The present invention relates generally to on-board diagnostic monitors for internal combustion engines such as misfire detectors, and more specifically to on-board adjustment of correction factors used by the diagnostic monitor.
One way to detect misfires in internal combustion engines is to measure crankshaft speed and observe fluctuations in speed. Detection of deviations in crankshaft velocities (as manifested by abnormal acceleration values) from expected normal crankshaft velocities is an indication of misfire. Deviations in acceleration are determined over nominally equal successive intervals of crankshaft rotation, referred to as "Profile-Ignition-Pick-Up intervals" (PIP intervals). A PIP signal is a digital signal received from a sensor, which detects specified positions of rotation of a crankshaft mounted wheel during engine rotation. PIP intervals are also known as combustion intervals, which are equal in length (but not necessarily phase) to the angular rotation between top dead centers of adjacent cylinders in the firing order.
Ideally, during normal operation, an engine will produce a series of PIP transition signals whose periods are inversely related to the average crankshaft velocity during a substantial portion of the power stroke for each of the cylinders in the engine. The crankshaft velocity will either remain constant (zero acceleration), increase (positive acceleration) or decrease (negative acceleration) depending on whether the engine is operating at steady state, accelerating or decelerating, respectively. For example, if a normal engine is operating under steady-state operation (no acceleration), then acceleration values of near zero are expected over successive PIP intervals. However, if a particular cylinder in an engine produces a sufficiently negative value during steady-state operation, then this occurrence will be interpreted as a misfire condition, since a zero value is expected as an output for all cylinders in a normal engine during steady state operation.
Accordingly, misfire detectors typically look for individual cylinders yielding acceleration values different from the local norm of all cylinders, where the local norm depends upon the operating condition (i.e., steady state, acceleration, or deceleration, etc.). The problem is that individual cylinders in normal engines tend to yield values of velocity that differ slightly from the local norm of all cylinders in a systematic manner according to cylinder number. In a normally operating engine, this systematic variation will interfere with the misfire detection system's ability to detect abnormal behavior due to misfire.
There are at least two sources of such cylinder-specific-irregularities. The first is discussed in U.S. Pat. No. 5,117,681 to Dosdall et al., incorporated herein by reference. Dosdall et al. deals with systematic irregularity arising from PIP spacing of the wheel. If the wheel which serves as the position encoder on the crankshaft is even slightly irregular in PIP-interval spacing (e.g., a few tenths of a degree difference), then a normal engine operating at constant true PIP-to-PIP velocity (steady state) will appear to be experiencing subtle velocity changes (hence, non-zero acceleration values). The velocity changes will appear to coincide with the particular cylinders associated with the irregular PIP intervals. Although the degree of impact that a given wheel error has on the acceleration calculation is strongly rpm-dependent, the error itself is fixed, and so it can be empirically determined at any operating condition, even deceleration.
Dosdall et al. prefer to employ defueled coast down when sensing encoder errors. The reason is to avoid uneven acceleration due to firing events during data collection, since with no fuel, all cylinders are unpowered. Application of the process specified in Dosdall et al to coast down data yields a set of values which indicates the actual PlP-interval spacings of the wheel relative to the nominal values (i.e., assumed equal spacing). There are only n/2 unique correction values derived in this manner, since crankshaft mounted wheels typically have half as many PlP-intervals as engine cylinders (n). Each individual correction value is used twice per engine cycle.
The second problem, discussed in U.S. Pat. No. 5,377,537, issued to James and incorporated herein by reference, is that even under normal operating conditions the crankshaft will produce different amounts of speed-up and slow-down because of the non-rigid (torsional) behavior of the crankshaft. Since the crankshaft is not rigid, it produces subtle, but reproducible oscillations (due to crankshaft flexing) in the PIP signal (systematic "noise"). This noise tends to camouflage true misfires and can cause an erroneous indication of one or more cylinders as having misfired even when engine operation is in fact normal. For example, cylinders at the front of a crankshaft might affect the speed of the crankshaft at the measuring point slightly differently than cylinders at the rear. The effects of crankshaft-torsional-flexing can occur when the power is cut-off (as in the Dosdall et al. patent), because inertial torques produce uneven motions (acceleration). In general, the effects of such torsional variations on the calculated acceleration values are most pronounced at higher engine speeds as manifested in typical engine data.
U.S. Pat. No. 5,377,537 discloses a method for removing the effects of torsional oscillations by applying correction factors to velocity measurements and using the corrected velocities (or accelerations) for misfire detection. Correction factors for each of the n PIP intervals of an engine cycle are generated for each of a multitude of engine operating conditions. Each set of n generated correction factors is stored in a table (a "look-up table" located in a memory unit) and has a corresponding unique address location. This table is determined empirically in the calibration of the engine type by a test engine. The correction factors are generated while the test engine is rotated at a fixed steady state operating point with combustion, where the operating point is typically a particular speed and load. The table is loaded in memory units of production vehicles. During operation, measured operational parameters from the engine (e.g., a particular speed and load) are used as an address to retrieve correction factors stored in the look-up table.
Although a test engine characterizes the expected torsional oscillations for a particular engine design reasonably well, production engines of that same design may exhibit individual variation and may also experience changes as the engines wear with use. Therefore, the effectiveness of a predetermined look-up table may be reduced over time unless it is adjusted or adapted for the changes.
Adaptation of correction factors for inaccuracies in the placement of position markers on a crankshaft position sensor is shown in U.S. Pat. No. 5,237,862 issued to Mangrulkar et al. Adaptive correction methods, however, must overcome several difficulties. The basis of adaptation is that the correction factors should be derived during a time when misfire or other malfunction is not present. Thus any adaptive learning of correction factors must be disabled under conditions of misfire. However, detection of a misfire depends upon the capability of the correction which itself depends upon the ability to detect a misfire, leading to a circular problem that adaptation cannot be achieved until adaptation is achieved. In addition, if updating of the learned correction factor is cutoff during conditions of misfire and if engine conditions are changing during that same period, the latest available correction factors may become inaccurate. Furthermore, it is possible that the adaptive correction may adapt to a gradual onset of power loss in a cylinder and thus become incapable of detecting an actual power loss that has reached the required threshold.